Bacteria and plants could help clean up toxic waste from gold mines – South African study

Highly toxic metals are leaking out of nine abandoned mines in Krugersdorp, 32km west of Johannesburg, South Africa. The metals include uranium, arsenic, cadmium, chromium and mercury.

This acid mine drainage – when water mixes with heavy metals in mines and then flows out into the environment – poses grave health risks. It can percolate through the soil in large quantities and end up in the groundwater, or get into rivers.

Acid mine drainage also dries out on the surface and then blows around as toxic dust. It can cause serious illness including cancer, kidney failure and asthma.

We are chemists who have studied mine pollution for decades, and look for solutions to prevent environmental damage. Driving through Krugersdorp, we’ve seen children playing on the mine dumps. We’ve also noticed dams in the area where people swim or hold baptisms. We decided to find a way for Krugersdorp’s mine dumps to be cleaned up.

We reviewed different acid mine drainage treatment technologies from around the world to determine which technologies would remove toxic metals and any other waste left behind after the removal. Our research concluded that a combination of existing treatments would be most effective in cleaning up the Krugersdorp sites. Specifically, using plants to suck the toxins up or bacteria to eat them.

This is the first time researchers have proposed some of the combinations of treatments to clean up acid mine drainage. This is significant because it means more toxic elements are being addressed and fewer are left behind.

Cleaning up

Together with a scientist from South Africa’s state-owned mineral research organisation, Mintek, we reviewed the existing clean-up methods and technologies.

We found that some available treatment technologies only treat certain pollutants and leave other toxic minerals behind. Others, when used for large scale cleanups, are very expensive and produce a lot of secondary waste (waste resulting from the treatment technologies), which then needs its own disposal process. This defeats the purpose.

We came to the conclusion that combining several technologies is the solution for Krugersdorp’s acid mine drainage.

One cleanup process is phytoremediation – when plants are grown on top of solid pollution and absorb toxins through their roots, removing them from the environment.

The other technology, developed by Mintek, is known as “cloSURE”. It uses anaerobic prokaryotic microorganisms – a type of bacteria that can grow in large quantities when oxygen is absent, for instance in lakes or manure. These tiny micro-organisms, when mixed with the acid mine drainage, remove all the metals and the sulfate from the waste.

Using both of these technologies in Krugersdorp would be effective for various reasons.

The cloSURE technology can treat acid mine drainage with high sulphate and metal content – the type of acid drainage in Krugersdorp. Meanwhile, the plant-based phytoremediation approach is very effective on solid dumps such as mine dumps because it can be grown over the mound, binding it together and preventing the waste from eroding and spreading out.

When both these technologies are applied in the same area, they will ensure that both liquid and solid mine waste is treated.

Finally, they’re also affordable technologies.

The government should put funds aside for this purpose, so that the residents are protected from the adverse effects of mine dumps which have existed for decades.

What’s polluting Krugersdorp

Gold mining began in Krugersdorp in 1887, over 100 years before South Africa’s National Environmental Management Act compelled mining companies to manage the waste they produced. The goal of acid mine drainage treatment is to produce treated water that can be reused in mining, irrigation, drinking or simply released back into the environment as clean water.

Before this law was enacted in 1998, mining wastewater was released into the environment without any treatment or management.

Krugersdorp communities, including farmers who use contaminated water downstream, have been affected by the acid mine drainage from the abandoned gold mines in the area.

We knew from our earlier research of the high concentrations of uranium, arsenic, cadmium, chromium and mercury (ranging between 0.3mg and 12mg per litre) in the soil of Krugersdorp mine dumps. These levels are far above the World Health Organization’s allowed limits for these minerals in wastewater and soils.

The Krugersdorp mine dumps are in an open space and during the windy season in July and August every year, residents further away breathe in this contaminated air. The white particles from the mine dumps (normally carrying toxic elements) form a cloud of dust that can be seen kilometres away during windy days.

Our research also found that the Cradle of Humankind’s calcium carbonate rocks are at risk from the untreated acid mine drainage. This is because the Tweelopiespruit stream drains out of the mining areas in Krugersdorp and runs through to the Cradle of Humankind, a world heritage site.

It’s imperative that cleanup technologies address this major environmental and health risk.

What needs to happen next

The Krugersdorp gold mine waste needs to be cleaned up urgently by the government. The technologies we’ve suggested are well-suited and can work fast. If this is not treated now, the impact of the contaminated water and dust on the health of the people living in the area will be much more severe. Government needs to allocate large amounts of money to clean up the mine dumps and acid mine drainage.

Our research reveals just one of the innovative ways in which acid drainage can be addressed. But more needs to be done to tackle the issue more broadly.

There’s a need for scientists to come up with cheap and efficient adsorbents – solids that can suck pollutants out of an area, such as activated carbon.

Local governments should build small acid mine drainage treatment plants. These should pilot cleanup solutions to see which are viable.

We also recommend that a global app be set up which can select the best combinations of technologies to treat specific acid mine drainage spills. All the treatment technologies used successfully across the world should be added into the application.

Researchers also need to conduct more studies into the extent of acid mine drainage pollution in communities close to mines. This data can be used to urge the government to act hastily to treat acid mine drainage and prevent future disasters.

Finally, there are also other clean up technologies which are useful for recovering valuable rare earth elements, such as copper or silver, from the water. If these metals can be recovered and used again, this will reduce the stress on the environment caused by mining.

Dr Gebhu Ndlovu, head of the Hydrometallurgy Division at Mintek, co-authored the research that this article is based on.